With the development of magnetic recording mediums in recent years, high-density recording and high electromagnetic properties have been demanded. Yet, at the same time, running properties must be maintained at a satisfactory level. The provision of a smooth magnetic layer surface has been examined and calendering has been carried out towards this end in order to obtain good electromagnetic properties. On the other hand, the surfaces of magnetic recording mediums have been appropriately roughened by incorporating fillers in the back layers thereof in order to improve running properties.
In video tapes prepared by incorporating fine particles of ferromagnetic powder, for example, the latest S-VHS video tapes, surface designs for high-density recording are required and spacing loss to magnetic head must be reduced to the greatest extent possible. However, conventional 1/2" base films are too rough and electromagnetic properties are poor due to insufficient smoothness.
As a method for solving the roughness problem while maintaining adequate running properties, there is proposed a method wherein a base provided with a flat surface on both sides (Ra as measured with contact type roughness tester being not more than 0.005 .mu.m) is used and a magnetic tape having a desired surface roughness is formed by manipulation of the compositions of the magnetic layer and the back layer. Base films for use in the production of such magnetic tape are described in JP-A-62-130848 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, since the base films are liable to be wrinkled or marred in the wind-up part of film-making stage, the yields of marketable base films are very low and, as a result, the manufacturing cost thereof is as high as 5 to 10 times that of 1/2 inch base films.
As alternative method for solving the above problems, dual surface films, wherein the face and the back of the films are different in surface roughness, are disclosed in JP-A-62-248131. However, it has been found that the surface roughness of the back layer side of the base film is too rough and hence the shape of the back layer is transferred to the surface of the magnetic layer during storage after winding-up, and, as a result, electromagnetic properties are lowered.
As described above, various attempts have been made to achieve high C/N (carrier to noise) for the video tapes by improving the surface profile of the magnetic layer. However, such prior attempts reveal a marked tendency to limit attention only to minimization of the Ra value of surface roughness (center line average surface roughness: JIS-B0601) with regard to the improvement of the surface profile of base film to be used. When the Ra value of the base film to be used is decreased, the output of the magnetic tape is improved. However, during handling in the manufacturing process, the integrity of the base is deteriorated and an increase in cost is incurred due to an avoidable lowering in yields. As can be appreciated from the foregoing discussion, the problem with regard to the optimization of the surface roughness of the base film cannot be easily solved by way of conventional proposed solutions. As a result, kinds of commercially available magnetic tapes for which high C/N is required are not considered to be fully satisfactory with respect to surface roughness. The description "kinds of magnetic tapes for which high C/N is required" as used herein refers to kinds of high band tapes such as S-VHS tapes.
The surface roughness of commercially available non-magnetic support film for conventional coating type magnetic tape was measured in the following manner. An area 250 .mu.m square was measured with an optical interference type three-dimensional roughness tester, and the difference between (a) an average height of the protrusions of 10 apices extracted in order of greatest height within an area 250 .mu.m square and (b) an average height of all apices within the 250 .mu.m square area (P.sub.10) was determined. On either side of the support film, P.sub.10 .gtoreq. was determined to be at least 38 nm.
When Such a conventional base film is used, large protrusions present on the surface of the base film are replicated to some extent on the surface of the magnetic layer coated on the base film. As a result, spacing loss between magnetic head and magnetic tape is increased due to the surface unevenness and it is impossible to achieve high C/N.
P.sub.10 of base films for vapor deposition is exceptional and as low as 1 to 10 nm. When 1/2 inch magnetic tapes are prepared by using such base films, sufficiently high C/N can be achieved as required for S-VHS video tape. However, the winding-up yields of the base films are very low, scratches are produced in the coating stage of the magnetic layer, and performance and yield are likely to be lowered. Hence, the production cost of the magnetic tapes which are the final products is increased.
It is demanded to provide a base film which can give high C/N and can be used for S-VHS without increasing the manufacturing cost of the magnetic tape.